Range hoods are commonly installed in an horizontal position, spaced above a cooking stove in order to remove the undesirable particles and odors inherent to the cooking operations. The volatile products are drawn into the hood by a draft of air induced by a fan or blower before being pushed through a discharge orifice. Two basic types of range hood discharge orifice have been used in the past namely ducted and ductless. With the ductless hood the air is first filtered and is then returned through a vent at the top of the hood. The more commonly used ducted hood is provided with an outlet orifice which is connected to a duct that discharges the air outside of the room and in most instances outside of the building.
With the ducted hood connected to a duct which discharges outside of the building, a flap assembly acting as a one way valve is commonly used at the final point of discharge to prevent outside air from entering the building when the blower assembly is inoperative. However, experience has clearly demonstrated that such flap assembly are often inefficient in keeping outside air from entering the building because variations in the pressure differential between air outside of the building and inside of the room where the hood is located cause the flap to rattle and open randomly. The variations in the pressure differential are induced by a variety of factors such as variations in the wind intensity, opening of doors inside the building, the use of heating devices and so forth. In geographical regions where there exists a temperature differential between the air outside of the building and the air inside the room where the hood is located, the flap assembly is thus considered a potential source of considerable energy waste. Furthermore, since it is located at the final point of discharge outside of the building it is difficult to maintain and service and thus losses even more of its efficiency as the outside elements such as rain, snow or the like degrade its structure and alter its efficiency.
Some range hoods such as the one disclosed in the U.S. Pat. No. 4,266,527 make use of a damper 42 which are centrally suspended. These dampers are noisy because they are constantly flipping whenever they are subjected to a small circulation of air. For this reason, Barnhart et al, in the above mentioned patent, proposes felt strips to prevent the metal to metal contact. Such dampers are not very effective as energy savers. Furthermore, the damper 42 as well as the damper 27 requires a longitudinal space along the duct because they rotate about a transversal axle. Such a space is not always available in duct for range hood. In some cases, a damper such as 27 is liable to obstruct the free circulation of air when the ducts are set at an angle.